Japanese maple

Identification and origin

Acer palmatum belongs to the soapberry family, Sapindaceae, and comes from Japan and Korea.

The opposite leaves, 2–5 in long, have five to nine pointed, toothed lobes, often deeply cut. Yellow, lime green or bright pink as they unfold, green, bronze or purple in summer depending on the cultivar, they turn yellow, orange or red in fall. In spring, small reddish-purple flowers, not showy, give way to paired winged samaras about 1 in long. Gray, muscular trunks carry layered, horizontal branches.

Growth is slow to moderate, about 10–15 ft in fifteen years. Depending on the cultivar, the plant stays a shrub of 3.5–6.5 ft for many years or becomes a small rounded tree of 10–24.5 ft, often wider than tall; the species can reach about 26 ft in fifty years. Among hundreds of cultivars, often grafted:

• Palmate-leaved forms: ‘Bloodgood’, a small tree with purple foliage that holds its color well through summer; ‘Osakazuki’, known for its fall color; ‘Sango-kaku’, an upright form whose young coral-pink shoots brighten the winter garden.
• Dissectum group (laceleaf maples): leaves with seven to eleven lobes cut to the base, a rounded and often cascading habit, rarely more than 12 ft tall, such as ‘Crimson Queen’ or ‘Inaba-shidare’.

Light

Partial shade or dappled light, about four to six hours of light a day, sheltered from strong winds. Where summers are cool it tolerates full sun; where they are hot it needs afternoon shade, because young foliage scorches if the soil dries out.

Purple-leaved forms need some sun to color well; green forms look best in partial shade; variegated and laceleaf forms dislike afternoon sun.

Watering

Keep the soil evenly moist, never soggy: Japanese maple tolerates neither drought nor standing water. Instead of following a schedule, check how moist the soil is and watch the foliage: brown, crinkled leaf edges often point to a lack of water, especially in hot, windy weather.

Keep a close eye on it during the first year after planting, then water deeply during hot, dry spells. Lawn grass under the canopy competes for water, so remove it. In containers the root ball dries much faster and should never dry out completely: in hot weather, daily watering may be needed.

Potting mix

Humus-rich, moist, well-drained soil, ideally slightly acidic. It tolerates clay and somewhat alkaline soils, but a pH that is too high can cause chlorosis.

Spread 2–4 in of organic mulch under the canopy in a ring, keeping it off the trunk, and renew it every twelve to eighteen months. An established tree in the ground usually needs no fertilizer; if growth slows, apply a slow-release fertilizer in spring, before the leaves emerge.

In containers, choose a large pot and a peat-free loam-based mix such as John Innes No. 2, or a peat-free acidic mix with one quarter coarse grit. After the first year, a granular fertilizer in late spring is enough.

Temperature

The tree is hardy in USDA zones 5a–8b, meaning down to about -20 °F, although shoots sometimes die back after a harsh winter. It tolerates heat, but not heat combined with drought.

Spring is its weak point: it leafs out early, and young leaves are vulnerable to late frosts and cold winds. Avoid low spots where cold air settles; when a cold night is forecast, cover small trees with frost cloth. In containers the roots are more exposed: raise the pot on feet and wrap it in burlap in winter.

Repotting

In the ground, plant from early fall through late winter. The root flare, where the trunk widens into the roots, should stay visible, with no more than 1 in of soil over the main roots. A tree planted too deep develops girdling roots that wrap around the trunk and can eventually kill it. Cut any roots circling the root ball.

In containers, repot about every two years into a slightly larger pot, in spring or early fall. To keep the same pot, root-prune every two or three years by removing about 2 in from the bottom of the root ball and a few slivers from the sides, then top up with fresh mix.

Pruning

Prune lightly: the tree naturally has a graceful shape.

• When: from midsummer, once the spring flush of growth is over, through midwinter. In late winter and spring, cuts bleed heavily.
• What: dead wood, shoot tips killed by winter, crossing branches; on a young tree, shoots growing inward or too upright.
• How: cut back to a side branch without leaving a stub, which would rot. Disinfect tools between trees.

Propagation

Seed: cultivars do not come true. Collect the samaras when they fall in autumn, remove the wings and sow right away in pots set in a cold frame or a sheltered spot outdoors: winter cold breaks dormancy and seeds germinate in spring. Stored seed needs cold, moist stratification in the refrigerator below 41 °F for four to twenty weeks.

Layering: in fall or early spring, the easiest method for a cultivar.

Grafting: the usual nursery technique, tricky for home gardeners.

Common problems

A single symptom can have several causes: brown leaf edges may come from drought, sun, wind, soggy soil or disease. Check the site and the soil before treating.

• Scorched leaves: sun, wind or drought, especially on laceleaf forms.
• Blackened shoots in spring: late frost.
• Dull fall color: not enough light.
• Verticillium wilt: leaves on one branch or one side suddenly wilt, and the wood shows green to black streaks. No treatment works: cut out affected branches and limit nitrogen.
• Anthracnose: brown blotches along the veins and dying twigs in rainy springs; rake up fallen leaves.
• Root rot: poorly drained soil.
• Slow decline: girdling roots on a tree planted too deep.
• Pests: aphids, scale, mites; in containers, vine weevil larvae that eat the roots.

Toxicity

No toxicity has been reported for dogs or cats. Still keep children and pets from chewing on it. Horse owners should be careful with maples in general: some species in the genus, above all sycamore maple and boxelder, contain hypoglycin A, which causes atypical myopathy in horses.